A metal detection apparatus is used to detect metal contamination in edible goods and other products. Modern metal apparatuses utilise a search head that comprises a “balanced coil system” typically with three coils that are wound onto a non-metallic frame. A transmitter coil located in the centre is energised with a high frequency electric current that generates a magnetic field. Two coils on each side of the transmitter coil act as receiver coils. Since the two receiver coils are identical and installed with the same distance from the transmitter coil, an identical voltage is induced in each of them. In order to receive an output signal that is zero when the system is in balance, the first receiver coil is connected in series with the second receiver coil having an inversed sense of winding. Hence the voltages induced in the receiver coils, that are of identical amplitude and inverse polarity are cancelling out one another in the event that the system, in the absence of metal contamination, is in balance.
As a particle of metal passes through the coil arrangement, the high frequency field is disturbed first near one receiver coil and then near the other receiver coil. While the particle of metal is conveyed through the receiver coils the voltage induced in each receiver coil is changed typically in the range of nano-volts. This change in balance results in a signal at the output of the receiver coils that can be processed, amplified and subsequently be used to detect the presence of the metal contamination in a product.
The signal processing channels normally split the received signal into two separate components that are 90° apart from one another. The resultant vector has a magnitude and a phase angle, which is typical for the products and the contaminants that are conveyed through the coils. In order to identify a metal contaminant, “product effects” need to be removed or reduced. If the phase of the product is known, then the corresponding signal vector can be reduced. Eliminating unwanted signals from the signal spectrum thus leads to higher sensitivity for signals originating from contaminants.
Methods applied for eliminating unwanted signals from the signal spectrum therefore exploit the fact that the contaminants, the product and other disturbances have different influences on the magnetic field so that the resulting signals differ in phase.
Distinguishing between the phases of the signal components of different origin by means of a phase detector allows obtaining information about the product and the contaminants. A phase detector, e.g. a frequency mixer or analogue multiplier circuit, generates a voltage signal which represents the difference in phase between the signal input, such as the signal from the receiver coils, and a reference signal provided by the transmitter unit to the receiver unit. Hence, by selecting the phase of the reference signal to coincide with the phase of the product signal component, a phase difference and a corresponding product signal is obtained at the output of the phase detector that is zero. In the event that the phase of the signal components that originate from the contaminants differ from the phase of the product signal component, then the signal components of the contaminants can be detected. However, if the phase of the signal components of the contaminants is close to the phase of the product signal component, then the detection of contaminants fails, since the signal components of the contaminants are suppressed together with the product signal component. In known systems, the transmitter frequency is therefore selectable in such a way that the phase of the signal components of the metal contaminants will be out of phase with the product signal component.
U.S. Pat. No. 8,841,903 B2 discloses the metal detection apparatus shown below in FIG. 1, which comprises a transmitter unit 2 that provides transmitter signals to a transmitter coil 21 that is inductively coupled to first and second receiver coils 3, 31, 32, which are connected to the input of a signal processing unit 4 that comprises a receiver unit 41 connected to a signal processor 42. The transformer unit 1 comprises a frequency generator 11 that provides an operating frequency fTX to the input of an amplifier stage 12, whose output is connected via a coupling transformer 13 to the transmitter coil 21 in transmitter unit 2. The output of the amplifier stage 12 is connected via a first switch bank 14 to a first tapping 141; 142; 143 and the transmitter coil 21 is connected via a second switch bank to a second tapping 151; 152; 153; 154 of the same transformer winding 131 of the transformer 13. The transformer winding 131 has a number of n winding coils between the first tapping 141; 142; 143 and a common potential and a number of n+m winding coils between the second tapping 151; 152; 153; 154 and the common potential. The transmitter coil 21 comprises a number of q winding coils and is connected via a third switch bank 23 to tuning capacitors 221, 222, 223 or combinations thereof thus forming a resonant circuit that is tuned to the operating frequency fTX. The ratio n+m/q of the winding coils of the transformer winding 131 and the winding coils of the transmitter coil 21 is selected such that the inductance of the transformer winding 131 is at least ten times higher than the inductance of the transmitter coil 21.
With this arrangement, the resonant circuit, which consists of the transmitter coil 21 and the selectable capacitors 221, 222, 223, can be tuned optimally and independently of other parts of the transmitter unit. Due to the difference in inductances, the transformer 13 is decoupled from the resonant circuit allowing individual optimization of the different parts of the transmitter.
The amplifier stage consists of a class A or B amplifier that can be selected to provide an output signal in a suitable voltage range, e.g. 20 Vpp.
For phase detection of the response signals the transformer 13 comprises a further transformer winding 132 having a first and a second tapping 1321, 1323 and a centre tapping 1322 arranged therebetween. The voltage appearing across the second winding 132, which is fed as a reference signal sREF to the signal processor 42, corresponds exactly to the signal appearing across the receiver coil 3 when no products P and/or contaminants C pass through the balanced coil system 21, 3. Hence, with the reference signal sREF changes of the received signal induced by products P or contaminants C can exactly be detected. Since the reference signal sREF is phase-locked to the transmitter signal sTX at the output of the power amplifier 12 changes in the response signal can accurately be detected.
FIG. 1 further symbolically shows a conveyor 8, on which products P, which may comprise contaminants C, are transferred through the transmitter coil 21 and the receiver coils 31, 32.
This advantageous circuit arrangement still has drawbacks. A Class A circuit amplifies signals with minimum distortion, but with low efficiency since the power transistor consumes current continuously even in the quiescent state. Amplifier efficiency is defined as the ratio of AC power input to the load divided by the DC power consumed by the circuit. When at or near maximum output power, the efficiency of a typical Class A amplifier is only 40%, about 10% less than its theoretical 50% maximum. With reduced output power, the efficiency drops accordingly.
Class AB circuitry avoids crossover distortion to a large extent and operates with reduced losses, since in the quiescent state, due to the applied biases to the complementary pair of transistors, only a small collector current is present. This circuit requires complementary amplification wings typically with a PNP and a NPN power transistor arranged as emitter followers. Providing different but complementary amplification wings requires different electronic elements and a different design for each amplifier wing and therefore considerable manufacturing efforts. Furthermore, Class AB amplifier stages, with a “push-pull” circuit typically deliver at the emitters of the complementary power transistors an output voltage that is applied to the load. In order to avoid a drop in voltage, which is not compensated for, the output voltage is applied directly to the load thus avoiding connecting cables.
Further, the Class AB circuitry does not deliver reference signals for phase detection, wherefore said additional winding 132 is required in the transformer 13 with the result of additional manufacturing costs.
Furthermore, the options to tune and adapt the resonant circuit, consisting of the transmitter coil and the tuning capacitors, to the frequency of the input signal are limited. Hence, the metal detection apparatus operates with a limited range of operating frequencies. It is an object to provide an improved metal detection apparatus that overcomes this limitation.